Slant six or v8 swap for use in a truck

remember that. Thats why i stuck with a /6 in my 85. every weight class has a couple of GVWs within. MY D150 is the lightest at 4800 gvw which wasnt gonna change no matter what engine was in it. or how much I beefed up springs and such. itws what THAT truck was originally rated for. I've had D150s that were rated at 6000gvw. Still a " 150" but more capable than my current one.
My 83 d250 could have came with 1 of 3, GVW packages. Mine was the middle at 7500. the lighter one was less than 7000, the heaviest one was like 8500. still all D250s. but one truck of a given weight class may bea "heavier" version than one that seems "just like it".